Escherichia coli are commonly found in the gut of both humans and animals. Most E. coli are considered symbiotic; however, pathogenic strains have been isolated that are associated with foodborne illness in people and animals e.g., pathogenic E. coli K88 and K99 affect swine and calves, respectively. Transmission of pathogenic E. coli occurs through fecal contamination of food or water, and is commonly associated with the consumption of under-cooked meat, raw milk, or contaminated vegetables.
Pathogenic E. coli includes the Shiga-toxin producing strains known as STEC. Shiga-toxin is named for its resemblance to the Shiga-toxin produced by Shigella dysenteriae. STEC infection can be asymptomatic, or include symptoms of fever, watery diarrhea, severe abdominal pain, hemolytic uremic syndrome (HUS) and even death, with more severe cases typically being reported in young children or the elderly. Enterohaemorrhagic E. coli (EHEC) are a subset of STEC, characterized by their ability to form attaching and effacing intestinal lesions. Cattle are the main reservoir for EHEC, the bacteria living asymptomatically in the cow intestine, although these bacteria have also been isolated from the intestinal tract of other domestic animals including sheep, pigs, goats, and dogs. These EHEC predominantly colonize the recto-anal junction of cattle, thereby increasing the risk of transmission to humans through fecal contamination. Numerous EHEC have been isolated including serotypes O111, O145, O103, O26, and O157. According to the Centers for Disease Control and Prevention, O157:H7 is the most common serotype that causing E. coli-linked food poisoning in the United States. The infectious dose is estimated to be as low as 10-100 bacteria. EHEC infections can be difficult to treat and some antibiotics actually worsen symptoms of an EHEC infection by inducing Shiga-toxin production and increasing the risk of HUS.
The introduction of antibiotics as therapeutics in the mid-1940s was an important advancement for medicine in terms of reducing human morbidity and mortality. The subsequent emergence of antibiotic resistant bacteria, however, indicates that bacteria adapt to antibiotic pressure. Resistance can be acquired and maintained within a population through horizontal transfer of resistant genes, and/or through selection for mutations that confer resistance. Unfortunately, the use of antibiotics is widespread and invariably selects for resistance as continual exposure to the drugs inhibit susceptible strains and allows resistant strains to emerge and dominate a population. Selection for resistance occurs for all bacteria exposed to antibiotics, not just the specific pathogens that are being targeted. For example, when enrofloxacin was used to treat E. coli infections in poultry, it simultaneously selected for resistance in Campylobacter jejuni, which is another important food-borne pathogen. The increasing prevalence of resistant bacterial pathogens threatens the effectiveness of currently available antibiotics and presents a difficult challenge in human and animal medicine. The development of novel strategies to control pathogenic bacteria is necessary to 1) combat infection by existing strains and 2) provide alternatives so that antibiotic use, and hence the emergence of resistant strains, can be decreased.
Some bacteria have developed the ability to inhibit other bacteria, and further characterization of how this occurs could be helpful in the design of new anti-bacterial strategies. For example, cell-cell inhibition mechanisms have been documented in the literature and range from contact-dependent inhibition (1, 20) to production of narrow-spectrum antimicrobial proteins called bacteriocins. Bacteriocins typically restrict the growth of closely related bacteria (reviewed in (28, 31)). E. coli produce numerous bacteriocins (31), classified as either colicins or microcins (2, 11). Colicins are high-molecular weight, whereas microcins are typically <10 kDa. Microcins can be either chromosomally or plasmid encoded, whereas colicins have only been found on plasmids (13, 29, 30). Colicin production is usually correlated with an SOS response to stress (22, 34) and release of the colicin typically occurs through cell lysis. Microcins are secreted from intact cells (8, 27). Bacteriocins have been identified that kill competitors through pore formation, nuclease activity, or by inhibiting protein synthesis (3, 23-25).
Sawant et al. recently described a novel bacterial inhibition phenotype whereby defined strains of E. coli from cattle are able to inhibit growth of other E. coli strains including several strains of enterohemorrhagic E. coli (EHEC) and enterotoxigenic E. coli (ETEC) (32). During in vitro competition assays, susceptible strains declined an average 4-6 log in population size relative to their expected population density when grown as monocultures. The inhibition phenotype was called “proximity-dependent inhibition” (PDI) because of the apparent need for inhibitor and susceptible strains to be located in close physical proximity for the phenotype to be observed. Two different E. coli strains were described as expressing this trait (PDI+); multidrug resistant E. coli-25 and antibiotic susceptible E. coli-264. E. coli-25 and E. coli-264 do not affect the growth of each other, indicating that immunity is either conferred actively through the presence of an immunity mechanism, or passively through the absence of a receptor ligand found on susceptible cells.
Certain characteristics of the PDI phenotype resemble that of microcin production. For example, inhibition is effective against closely related species; PDI is not dependent on an SOS response; and production presumably does not kill the inhibitor strain (32). Nevertheless, microcins are soluble proteins and when Sawant et al. (32) employed a split-well experiment they demonstrated that close cell-cell proximity is required for the PDI phenotype to function. These findings suggest that the inhibition mechanism is not due to a soluble molecule unless the concentration is so low as to require close proximity to be effective (32).
The initial report of PDI provided a detailed description of the phenotype and a similar phenotype has been described between Bibersteinia trehalosi and Mannheimia haemolytica (4). Nevertheless, the exact mechanism of PDI and requisite genes for inhibition and immunity were not known at the time that the PDI was originally described. Progress in this field could aid the development of strategies to combat the emergence and spread of pathogenic bacteria, and to provide treatments for infection with pathogenic bacteria.